Jian Lu

Title: Response of the zonal mean atmospheric circulation: global warming vs. El Nino

The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). In contrast to the strengthening and contraction of the Hadley cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley cell weakens and expands poleward, and the jets move poleward under a warmed climate, despite the ''El Nino-like'' enhanced warming over equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and mid-latitudes in comparison to the El Nino.

A working hypothesis is proposed from analyzing the space-time spectra of eddy activity (momentum) flux under El Nino versus global warming conditions. While the response to ENSO is consistent with the refraction of midlatitude eddies due to subtropical wind anomalies, the internal variability of the annular modes marks a change in the eastward propagation speed of midlatitude eddies. In response to global warming, the dominant eddies exhibit a trend towards faster eddy phase speeds in both hemispheres, in a manner similar to the positive phase of interannual internal variability. These diagnoses suggest that the expansion of the Hadley cell and the annular mode trend due to greenhouse gas increase may be rooted in extratropical processes especially in the upper troposphere/lower stratosphere, rather than being forced from the deep tropics.